Production of powders of metal, alloy or other electrically conductive material



Dec. 24, 1963 A. s. DARLING PRODUCTION OF POWDERS OF- METAL, ALLOY 0R OT ELECTRICALLY CONDUCTIVE MATERIAL 3 Sheets-Sheet 1 Filed Dec. 4, 1961 FiG.l

LE ogcgcgfi m Dec. 24, 1963 A. s. DARLING 3,115,569

PRODU ION POWDERS OF METAL, ALLO R OTHER LLY DUCTIVE LEC CA CON MATER Filed Dec. 4, 1961 3 Sheets-Sheet 2 FIGZ 15c 756i 756 Q ,lfjc 75d 16 750 I l 1% K k wq 3 12 5 FIGS aimx 3% Dec. 24, 1963 A s. DARLING 3,115,569

PRODUCTION OF POWDERS OF METAL, ALLOY OR OTHER ELECTRICALLY CONDUCTIVE MATERIAL FiI'ed Dec. 4, 1961 3 Sheets-Sheet 3 l I i o o 8 L?! o g g o 0 ll 2 o o o o 0 o 9 Q I 3,115,569 PRGDUCTEUN 6F PUWDERS F METAL, ALLOY 0R Q'illlER ELECTRICALLY QGNDUCIIVE MA- TERIAL Alan Sydney Darling, Northwood, Middlesex, England, assignor to .lohnson, Mattlrey & Company Limited, London, England, a British company Filed Dec. 4, 1961, Ser. No. 156,591 Claims priority, application Great Britain Dec. 14, 1960 12 Claims. (Cl. film-6%) This invention relates to improvements in and relating to the production of powders of metals, metallic alloys or other electrically conductive material, and is more particularly, but not exclusively, concerned with the production of fine or flake powders, such as fine gold or silver powders for use in preparation of ceramic transfers or other ceramic decorating materials.

Various methods have been proposed, or are at present in use, for making metal powders. The most usual method is that of chemical reduction of a reducible compound of the required metal. Such a method is, however, costly and reproducibility of particle size and texture of the powders from batch to batch is difficult to attain.

It has also been proposed to produce metal powders electrolytically. For example, a silver anode and an inert cathode may be connected in an electrolytic circuit in an aqueous solution of an electrolyte and the silver deposit formed on the cathode continuously scraped from its surface. Such a method is complicated by the necessity for careful control at all stages if satisfactory results are to be obtained and is more suitable for producing a fairly coarse crystalline powder.

It is also old in the art to produce fine metal or alloy powders, particularly colloidal particles of silver and gold, by striking an are or series of arcs between electrodes, one or both of which is/are formed of the metal to be particulated, both submerged in a dielectric liquid, for example, water, in a suitable receptacle, the are or arcs acting to disintegrate the metal of the electrodes, the metal particles thus produced becoming dispersed in the dielectric or falling to the bottom of the receptacle for subsequent collection.

During the course of experiments carried out by the applicant with the object of overcoming the disadvantages inherent in the existing methods of powder production, it occurred to him that the known method of disintegrating metals by electroerosion or spark-discharge might constitute an answer to his problem. By this method, the surface of a metal article is eroded by means of a spark discharge from the metal as an anode to another electrode in a flowing dielectric medium, the eroded material being continuously removed from the area of the discharge by the flowing dielectric, and it seemed reason-able to assume that, since the eroded material, which is regarded as waste and must be removed, is in the form of small particles, the spark-erosion method of disintegration might be adaptable for what may be termed the converse operation in which the desired end product is not an article which has been treated by electro-erosion, but the actual eroded material, namely, powder-like particles of metal or alloy. The method, which is simple and efiicient and readily controllable, is thus particularly suitable for the production of fine metal or alloy powders.

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Further experiments confirmed this surmise and it was found that metal and other powders could, in fact, he successfully produced on an economical scale by suitable adaptation of the electro-erosion technique.

The principal object of this invention, therefore, is to provide a simple and eificient method of making powders of metals, metallic alloys or other electrically conductive materials.

Another object is to provide an improved method of producing metal or alloy powders by electro-erosion which is simple and eflicient.

A further object of the invention is to enable fine metal or alloy powders of sub-micron particle size to be readily produced at a high rate with accurate reproducibility from batch to batch.

A still further object of the invention is to provide a method of making metal or alloy powders by electroerosion procedures in which maximum use is made of the starting materials, and wastage is reduced to a minimum.

Broadly, these and other objects are attained in accordance with my invention by establishing an electric circuit between two relatively stationary electrodes, or a plurality of electric circuits, each between two of a plurality of relatively stationary electrodes, through the medium of a bed of closely-packed grains or other regularly or irregularly shaped pieces or scrap of the metal "or other material to be eroded, wetted by a suitable liquid dielectric, agitating the grains or pieces to cause repeated momentary mutual separation thereof to permit sparking between the individual grains or pieces with resulting erosion thereof, and collecting the particles so formed in the form of powder.

Agitation of the bed of closely-packed grains or pieces may be effected mechanically, for example, by mechanical vibration of the aforesaid electrodes or by fluidising the bed of grains or pieces by causing circulation of the liquid dielectric through the bed. Advantageously, however, a combination of these two procedures may be employed.

The electrodes may be in the form of elongated bars, the bars being arranged side by side in spaced parallel relation to one another in a suitable framework; or particularly in the case where more than two electrodes are used, they may take the form of a plurality of rows of spaced upstanding studs. Any desired number of pairs of such bar or stud electrodes, each pair being supplied with its own current, may be employed, the number depending on the quantity of powder to be produced in a given time.

The electrodes are preferably made in the form of bars of semi-circular or arc-shaped cross-section or in the form of hemispherical studs, the former being advantageously employed in the production of flake powders and the latter when fine powders, for the use in the production of ceramic transfers, are required. In the case of the use of stud-like electrodes, it will be found that it is not necessary for each anode to have its own independent cathode, but that a plurality of said anodes may be employed with a common cathode.

It will be appreciated that the anodes should, preferably, be formed of the same material as that of the powders being produced in order to avoid the possibility of contamination of the powder product.

The method of the invention may be successfully applied to the production of single metal powders or of alloy powders comprising two or more different metals.

In order that the invention may be fully understood, one embodiment thereof will now be described by way of example, by reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 is an elevational view of an apparatus embodying the invention and suitable for use in carrying out the method of the invention.

FIGURE 2 is a plan view of the electrode-carrying tray showing one form. which the electrodes may take,

FIGURE 3 is a vertical section of FIGURE 2 taken on the line 3-3 of FIGURE 2,

FIGURE 4 is a similar view to FIGURE 2 but showing a modified form of electrodes,

FIGURE 5 is a vertical section of FIGURE 4 taken on the line 55 of FIGURE 4, and

FIGURE 6 is a diagram of a suitable electrical circult.

Referring now to the drawings, and first to FIGURE 1 thereof, 1 denotes a container, of suitable material, such as stainless steel, for liquid dielectric 2., the said container 1 having an outlet 3 to which is coupled one limb i of a U-shaped pipe line 4, the other limb 4 of which is connected to an axial flow pump 5, from which extends a further pipe 6, the upper closed end 6 of which is supported in a framework '7 and from which extends a return pipe 8 leading to, and opening into, the said contalner 1 above the level 2 of the liquid dielectric 2.

The base of the U-pipe 4 is formed with an outlet 9 incorporating a butterfly valve it the purpose of which will appear hereafter.

As will be readily appreciated by means of the above arrangement the liquid dielectric 2 may be continuously circulated through the container ll.

Suspended in the container 1, by means of a cable 11 attached to a suitable support, not shown, so as to lie above the level 2 of the liquid dielectric 2 and below the level of the return pipe 8, is a framework 12, supporting a tray r13, made of suitable plastic material. Coupled to the suspension means is a mechanical vibrator 14 adapted to cause vibration of the framework 12 in the operation of the apparatus. The vibrator 14 should be such as to apply vertical linear vibrations only in order to prevent migration of the grains from one end of the tray 13 to the other' To the inner surface of the tray13 are bolted a plurality of spaced rows, eight such rows being shown in FIGURE 1, of electrodes, designated generally by the numeral l5, each IO'W comprising either a single bar of semi-circular cross secion as illustrated in FIGURES 4 and 5 or a plurality of spaced hemi-spherical studs, bolted to the tray 13 by means of bolts 16 and nuts 17, as illustrated in FIGURE 1 and in FIGURES 2 and 3. In the former case, each pair of adjacent bar electrodes I15 is connected in a separate electric circuit, one electrode thus constituting an anode and the other a cathode, such as 15* and 15 A suitable circuit is illustrated in FIGURE 6, to be referred to hereafter.

In the case of the use of individual stud electrodes 15, all the electrodes 15 in one row of any pair of rows may form the anodes and all the electrodes 15 in the other row of the said pair may form the cathodes of the pertaining electric circuits. Or, as clearly shown in FIG- URES 2 and 3, each anode 15 need not have its own independent cathode 15 but a plurality of anodes may be employed with a common cathode, which is included in each of the pertaining electric circuits. The anodefo-rming electrodes are preferably formed of the same material as that of the grains or pieces to be eroded.

18 indicates a closely-packed mass of grains of metal,

in this instance gold, which is distributed over the bottom of the tray 13 so as to cover the electrodes 15 and also the spaces therebetween.

In carrying out the method of the invention with the use of the above described apparatus, shown in FIGURE 1, for the production of fine gold powder, for use in the production of ceramic transfers, the grains of gold 18 are first distributed on the tray 13 as above indicated.

The electric circuits of the respective pairs of electrodes 15 and 15 are then energised, the circuits being completed through the closely-packed gold grains 18.

The vibrator 14 is then set in motion and at the same time the pump 5 is started. As a result, the tray 13 is vibrated and causes agitation of the grains 18 and momentary mutual separation thereof. This agitation is further assisted by the circulation of the liquid dielectric 2/ under the action of the aforesaid pump 5, from the container I through the U-shaped pipe 4 4 the pipe 6 and return pipe 8 back to the said container.

This momentary separation of the metal grains 18 has the effect of causing spark discharges to occur between the individual grains l3 and between individual grains and the electrodes 15. The grains 18 are thereby disintegrated and the fine gold powder, thus produced, is washed over the top of the tray 13, the relatively coarser particles falling by gravity to the bottom of the container 1, along the limb 4a of the U-pipe 4 and past the open valve 19 of the outlet 9 to a suitable collecting vessel. The bulk of the powder will, however, be too fine to settle in this manner but will remain in suspension in the liquid dielectric and will circulate with this liquid until the pump 5 is shut down, whereupon it will also fall by gravity past the valve 10 into the collecting vessel.

If flake powders are to be produced, it will be found preferable to employ bar electrodes, such as shown in FIGURES 4 and 5.

FIGURE 6 illustrates a suitable electrical circuit for use with each pair of electrodes, the circuit including two capacitors 19 arranged in parallel. In practice, however, any desired number of capacitors may be connected either individually or in parallel, so that any value of capacitance, between 1 microfarad and, say, 15 microfarads may be selected by steps of 1 microfarad. The circuit is operated from a 110 volt direct current supply source 20.

Whilst in the above, the invention has been described in connection with the production of fine and flake gold powders, it will be readily understood that powders of other metals, such for example, as silver, or alloy powders, composed of two or more different metals, such, for example, as brazing alloy powders, may also readily be produced in accordance with the teachings of the invention. The invention will also be found equally applicable to the production of powders of electrically conductive materials other than pure metals or alloys.

The material to be disintegrated, moreover, need not be in the form of grains, as described above, but satisfactory results may also be obtained by the use of scrap or of other regularly or irregularly shaped pieces.

Furthermore, whilst in the above, an embodiment of the invention has been described by way of example, it is to be understood that modifications may be made to the details thereof without departing from the scope of the invention. For example agitation of the material being disintegrated may be effected only by vibrating the tray or only by circulating the liquid dielectric. In the latter case, the electrodes and the bed of grains may, if desired, be immersed in the liquid dielectric.

As will be readily appreciated, the particle size of the powder produced may range from relatively coarse powder to powder of micron or sub-micron size as required in dependence on the selected capacitance and electrical parameters of the electrical circuit or circuits. In general, the higher the capacitance of the, or each, circuit, the coarser will be the resulting particles. For example, silver powder of micron size can be obtained with a current of 1.25 amps. and a capacitance of 4 ,ufarads at the rate of 5.8 grams per hour using 110 volts D.C. With a capacitance of ,ufarads and a current of 10 amps,

coarser powder in the size range of -15 microns is producible at the rate of 118 grams per hour.

The rate of production of powders of any chosen particle size will depend on the number of pairs of electrodes employed, since the quantity of powder of any chosen particle size produced by sparking between one pair of electrodes over a given period of time is predetermined by the chosen electrical characteristics of the circuit.

The invention, moreover, is intended to include within its scope, powders of any chosen particle size, ranging from relatively coarse to submicron sizes, when made from metals or alloys or other electrically conductive material by the method of, or with the use of apparatus in accordance with, the invention.

What I claim is:

1. A method of making powders of material selected from metallic, metallic alloy and other electrically conductive materials by electro-erosion which comprises providing at least one pair of relatively stationary electrodes, arranging on said electrodes, in such manner as to cover said electrodes and the space between at least said pair of electrodes, a closely-packed mass of pieces of said material wetted by a liquid dielectric, establishing an electric circuit between at least said pair of electrodes through the medium of said mass of material, the characteristics of said circuit being such as to produce short duration pulses of energy, agitating said mass to cause repeated momentary mutual separation of the constituent pieces of said mass, whereby spark gaps are formed between constituent pieces on which the pulsed energy can operate to cause erosion of said pieces and collecting the particles so formed in the form of powder.

2. A method according to claim 1 wherein the constituent pieces of said mass are in the form of grains.

3. A method according to claim 1 wherein the constituent pieces of said mass are in the form of scrap.

4. A method according to claim 1 in which the potential of the circuit is 110 volts direct current.

5. A method of making powders of material selected from metallic, metallic alloy and other electrically conductive materials by electro-erosion which comprises providing at least one pair of relatively stationary electrodes, arranging on said electrodes, so as to cover said electrodes and the space between at least said pair of electrodes, a bed of closely packed grains of said material immersed in a liquid dielectric, establishing an electric circuit between at least said pair of electrodes through the medium of said bed, the characteristics of said circuit being such as to produce short duration pulses of energy, mechanically vibrating said electrodes to cause agitation of said bed and repeated mutual separation of the constituent rains, whereby spark gaps are formed between said constituent grains over which the pulsed energy can operate to cause erosion of said grains, and collecting the particles so formed in the form of powder.

6. A method of making powders of material selected from metallic, metallic alloy and other electrically conductive materials by electro-erosion which comprises providing at least one pair of relatively stationary electrodes, arranging on said electrodes, so as to cover said electrodes and the space between at least said pair of electrodes, at bed of closely-packed grains of said material, establishing an electric circuit between at least said pair 'of electrodes through the medium of said bed of grains, the characteristics of said circuit being such as to produce short duration pulses of energy, agitating said bed of grains by circulating thereover a liquid dielectric to cause repeated mutual separation of the constituent grains thereof whereby spark gaps are formed between said constituent grains over which the pulsed energy can operate to cause erosion of said grains, and collecting the particles so formed in the form of powder.

7. A method of making metallic powders by electroerosion which comprises providing a plurality of pairs of relatively stationary electrodes, arranging on said electrodes a bed of closely-packed grains of metallic material, establishing an electric circuit between each pair of said plurality of pairs of electrodes through the medium of said bed of grains, the characteristics of said circuit being such as to produce short duration pulses of energy, simultaneously mechanically vibrating said electrodes and fiuidising said bed of grains by circulation of a liquid dielectric thereover, said liquid circulation causing agitation of said grains and repeated mutual separation thereof whereby spark gaps are formed between said grains over which the pulsed energy can operate to cause erosion of said grains and collecting the metallic particles so formed in the form of powder.

8. A method according to claim 7 wherein the metallic material consists of silver.

9. A method according to claim 7 wherein the metallic material consists of gold.

10. A method of making powders of material selected from metallic, metallic alloy and other electrically conductive materials by electro-erosion which comprises providing a framework including a tray, mounting in said tray a plurality of rows of electrodes arranged side by side in spaced parallel relation to one another, suspending said tray in a container containing a supply of liquid dielectric in such manner that said electrodes are immersed in said dielectric, arranging on said tray, in such manner as to cover said electrodes and the spaces therebetween a closely-packed mass of pieces of said material, establishing a separate electric circuit between successive pairs of said adjiacently disposed electrodes through the medium of said mass of material, the characteristics of said circuit being such as to produce short duration pulses of energy, circulating said liquid dielectric through said mass to cause agitation of the constituent pieces thereof and repeated momentary mutual separation of said pieces whereby spark gaps are formed between said constituent pieces over which the pulsed energy can operate to cause erosion of said pieces, and collecting the particles so formed in said container in the form of powder.

11. A method of making metallic powders by electroerosion which comprises providing a framework including a tray, mounting in said tray a plurality of rows of electrodes, arranged side by side in spaced parallel rela- -tion to one another, suspending said tray, by means including a mechanical vibrator, in a container containing a supply of liquid dielectric, so as to lie above the level of said dielectric, arranging on said tray, in such manner as to cover said electrodes and the spaces therebetween, a bed of closely-packed grains of metallic material, establishing a separate electric circuit between successive pairs of said adjacently disposed electrodes through the medium of said bed of grains, the characteristics of said circuit being such as to produce short duration pulses of energy, simultaneously vibrating said electrode-carrying tray by means of said mechanical vibrator, and fluidising said bed of grains by circulating said liquid dielectric through said bed, which circulation also causes agitation of the constituent grains and momentary mutual separation thereof whereby spark gaps are formed between said constituent grains over which the pulsed energy can operate to cause erosion of said grains, and collecting the particles so formed in said container in the form of metal powder.

.12. Apparatus for making powders of material selected from metallic, metallic alloy and other electrically-conductive materials by electro-erosion comprising, in combination, a suspended framework, including a tray, at least one pair of cooperating electrodes mounted in said tray in spaced relation to one another, said electrodes being included in, and forming an anode and cathode respectively in, an individual electric circuit adapted normally to be closed by a mass of the material to be eroded disposed on said tray so as to cover said electrodes and the space therebetween, the characteristics, of said circuit being such as to produce short duration pulses of energy,

means for circulating liquid dielectric through the mass of material on said tray and a mechanical vibnator associated with said tray and operable to impart vertical linear vibrations to said tnay, whereby in the use of the apparatus actuation of said circulating means and mechanical vibrator ca-uses momentary muwal separation of the constituent pieces of said mass of material, thereby forming spark gaps between said constituent pieces over which the pulsed energy can operate and cause erosion of said pieces to form powder.

References Cited in the file of this patent UNITED STATES PATENTS Von Ediger Mar. 24, Rudiortf Sept. 2, Stirnemann et al Mar. 24, Johnson Mar. 27, Tanaka Oct. 23,

FOREIGN PATENTS Great Britain June 10, 

1. A METHOD OF MAKING POWDERS OF MATERIAL SELECTED FROM METALLIC, METALLIC ALLOY AND OTHER ELECTRICALLY CONDUCTIVE MATERIALS BY ELECTRO-EROSION WHICH COMPRISES PROVIDING AT LEAST ONE PAIR OF RELATIVELY STATIONARY ELECTRODES, ARRANGING ON SAID ELECTRODES, IN SUCH MANNER AS TO COVER SAID ELECTRODES AND THE SPACE BETWEEN AT LEAST SAID PAIR OF ELECTRODES, A CLOSELY-PACKED MASS OF PIECES OF SAID MATERIAL WETTED BY A LIQUID DIELECTRIC, ESTABLISHING AN ELECTRIC CIRCUIT BETWEEN AT LEAST SAID PAIR OF ELECTRODES THROUGH THE MEDIUM OF SAID MASS OF MATERIAL, THE CHARACTERISTICS 